Trace metal removal is a critical step in the semiconductor fabrication process. Trace metal contamination has been shown to be a factor in limiting device yields. Metallic contamination impacts device performance by degrading gate oxide quality, leakage current density and minority carrier lifetime. Currently, most metal removal is achieved with wet chemical cleaning techniques such as the "RCA" clean and liquid phase HF. These processes have served the industry well for many years, and a recent resurgence of interest in cleaning techniques has led to a further refinement of wet cleaning technology. However, there is some concern that wet cleaning techniques will have increasing difficulty in cleaning future generation devices, with ever shrinking dimensions, because of surface tension effects. The predicted shift to single wafer, clustered processing also raises concern since wet cleaning processes are typically batch processes and are incompatible with the vacuum environment in which clustered process are performed.
Gas phase, or "dry" wafer cleaning processes can potentially replace wet cleaning and be more effective in a reduced device dimension clustered processing environment. Developing dry etching process for etching metals and removing metal contaminants, particularly copper nickel and iron or oxidized compounds thereof, is therefore desired. In evaluating such processes surface termination, roughness, and residues are the few major factors used to assess the feasibility of dry cleaning processes.
Silicon or germanium etching using unactivated fluorine interhalogens such as ClF.sub.3 is reported in GB 1180187 and lbbotson, et al., Appl. Phys. Lett. 44, 1129 (1984). Etching of silicon, tantalum, and certain tantalum compounds, selectively relative to silicon oxide, tantalum oxide and silicon nitride, is reported in U.S. Pat. No. 4,498,953; Ibbotson, et al, "Plasmaless Dry etching of silicon with fluorine-containing compounds," J. Appl. Phys., 56, pp 2939-2942, (1984); and Ibbotson, et al, "Selective interhalogen etching of tantalum compounds and other semiconductor materials," Appl. Phys. Lett, 46, Apr. 15, 1985, pp 794-796.
Several authors at Fujitsu Laboratories, Ltd., have produced publications describing a UV/Cl.sub.2 process for cleaning metal contamination from silicon wafer substrates and UV/F.sub.2 /Ar and UV/F.sub.2 /H.sub.2 processes for etching silicon oxide. These publications include T. Ito, "Wafer cleaning with photo-excited halogen radical," Proceedings--Institute of Environmental Sciences, 1991, pp 808-813; Aoyama et al, "Removing native oxide from Si(001) surfaces using photoexcited fluorine gas," Appl. Phys. Lett., 59, November 1991, pp 2576-2578; Aoyama et al, "Silicon Surface Cleaning Using Photoexcited Fluorine Gas Diluted with Hydrogen," J Electrochem. Soc., 140, 1704-1708 (1993); and Aoyama et al, "Surface Cleaning for Si Epitaxy Using Photoexcited Fluorine Gas," J Electrochem. Soc., 140, 366-371 (1993); and U.S. Pat. No. 5,178,721. In their published work these authors observed that the UV/F.sub.2 /Ar process, at the temperatures they investigated, did not remove iron and that the UV/Cl.sub.2 process required substrate temperatures in excess of 140.degree. C. to remove iron. It has also been reported that these authors observed that copper was not removed by their UV/Cl.sub.2 process unless a bare silicon wafer was also placed in the reactor.
In Sugino, et al, Proceedings ISSM'95 262-265 (1995), a report published subsequent to the invention of the present invention, Fujitsu investigators report that the UV/Cl.sub.2 process is also not completely effective for removing metals from SiO.sub.2 substrates; that iron is removed from Si and SiO.sub.2 substrates using UV/SiCl.sub.2 and substrate temperatures in excess of 350.degree. C.; and that on Si substrates the efficiency of removal is increased with a small addition of Cl.sub.2.
In U.S. Pat. No. 5,350,480 there is described an apparatus for generating directed beams containing thermally excited, electrically neutral gas species. The apparatus is reported to be capable of removing metal contaminants from semiconductor material substrates when a ClF.sub.3 beam is used. Other devices for producing such beams are described in references cited in U.S. Pat. No. 5,350,480. Whether produced by the apparatus of U.S. Pat. No. 5,350,480 or otherwise, beams of thermally excited, electrically neutral gas species (thermal beams) are considered interesting due to their ability to achieve highly anisotropic and almost damage-free etching profiles. The present inventors, however, have found that while dispersed transition metals, such as copper, nickel and iron, or their oxides, can be removed from a silicon surface maintained at ambient temperature using ClF.sub.3 alone, discrete islands of copper are not effectively removed under the same process.